1. Field of the Invention
The present invention relates to a soldering method. More specifically, the present invention relates to a method of electrically and mechanically connecting a substrate such as a rigid substrate and a flexible substrate, or a method of electrically and mechanically mounting on a substrate a connected member such as an IC (integrated circuit) chip, a discrete element and so on.
2. Description of Related Art
In recent years, there have been growing demands, in the installation of electronic products, for techniques of connecting and mounting components which are to be put in place at microscopic intervals. In the past, flux has been used in the soldering (which hereinafter implies an electrical connection by means of soldering and brazing). The flux has to be removed by cleaning after a connecting operation is finished since it is corrosive. In view of the environmental problems, however, there has been increasing restrictions imposed on the use of cleaning agents which contain CFCs (chlorofluorocarbons). A cleaning operation is sometimes difficult to be performed in the case of a soldering operation which has to be executed inside a package. Consequently, attempts have been made to implement soldering operations dispensing with cleaning operations by using low active flux which does not require the cleaning operation, or to perform a soldering operation in an inactive atmosphere without using flux if it has to be executed in a location which does not allow a cleaning operation. In the cases like these where low active flux is used or soldering operations are performed without using flux, it is difficult to secure a sufficient area for connection due to the effect of an oxide film on the surface of solder, which has given rise to such problems as the soldering being insufficient in strength and the connection being low in reliability, or voids occurring inside solder.
As an example of a soldering which will solve such problems as mentioned above, there is disclosed in Japanese Patent Laid Open No. 6-226485 a method in which a soldering is performed with a solder alloy coated with paraffin wax on its surface. Paying attention to the fact that the voids are caused by an oxide film, the technique has employed a method of covering solder with paraffin wax so as to prevent an oxide film from being grown on the surface of the solder.
Although the above-mentioned technique shows a method for preventing an oxide film from being formed, and for keeping and using solder under a condition in which there is no penetration of water, but it does not deal with soldering which has to be performed under a condition in which there is an oxide film formed on solder. In other words, soldering cannot be executed unless terminals have been solder-plated in advance as in the case of connections between terminals, and in the event of solder surface being oxidized, solder coated with paraffine wax cannot be used for connections between terminals.
It is an object of the present invention to provide a method of connecting or mounting a base member and a connected member, which can secure for them a reliable connection and an excellent insulation.
To solve the above-mentioned problems, a soldering method, which comprises a step of disposing a main connecting material composed of a soldering or brazing material and an auxiliary connecting material of an insulating material between a base member and a connected member; and a step of heat treating the main connecting material to be fused while at the same time causing the auxiliary connecting material to be cubically expanded or vaporized, can be employed. According to this, the auxiliary connecting material is subjected to cubical expansion or vaporization during the heat treating, the cubical expansion or vaporization of the auxiliary connecting material causes an oxide film grown on the surface of the main connecting material to be destroyed, and thus the base member and the connected member are electrically and mechanically joined through the main connecting material.
Accordingly, unlike flux, the auxiliary connecting material physically disperses the oxide film on the surface of the main connecting material without chemically dissolving it, which makes it possible for the base member and the connected member to be electrically and mechanically joined securely. Herein, in case the auxiliary connecting material comprises an insulating material having no functional group, the insulation quality thereof is prevented from deteriorating after the soldering operation, and thus it will not cause the insulation quality around an electrode joint to be reduced even if it is left in the periphery of the joint.
The main connecting material may be coated on at least one of connection surfaces of either the base member or the connected member. In this case, as the auxiliary connecting material, a material capable of vaporizing around at the melting point of the main connecting material may be selected to cause the auxiliary connecting material to be vaporized during the heat treating step. By means of this, the oxide film can be destroyed during the heat treating step to electrically and mechanically connect the base member and the connected member with high reliability, and the auxiliary connecting material can be vaporized away after soldering as well.
Also, as the auxiliary connecting material, a material mainly composed of such material whose boiling point is greater than the melting point of the main connecting material and whose vapor pressure is {fraction (1/100)} or over of the external pressure at a maximum temperature during the heat treating step may be selected to cause the auxiliary connecting material to be cubically expanded during the heat treating step. By means of this, the auxiliary connecting material existing around the joint portion will get into a boiling condition, undergoing an abrupt cubical expansion. The energy generated by this cubical expansion will physically disperse and destroy the oxide film which is staying as formed without being dissolved on the surface of the main connecting material, thereby causing the non-oxidized section to be exposed to electrically and mechanically connect the base member and the connected member with high reliability.
Furthermore, it may be possible for the main connecting material and the auxiliary connecting material to be kneaded and formed into a paste-like condition to be applied to the joint portion between the base member and the connected member. Besides enabling the base member and the connected member to be electrically and mechanically connected in a good condition, the formed paste also allows the base member and the connected member to be fixed tentatively through the paste, which makes it easier for them to be mounted.
Furthermore, the use of hydrocarbon for the auxiliary connecting material may fully vaporize the auxiliary connecting material at a temperature that the main connecting material is fused. More preferably, the hydrocarbon must be alkane, alkene or alkyne.